Carbohydrate metabolism in a heat-girdled maize source leaf

Heat-girdling in maize leaves was used for blocking assimilate export and producing alterations in carbohydrate metabolism. A rapid drop (< 24 h) in CO2 fixation and a large increase of sucrose and starch content were observed. Reduction in photosynthetic assimilation was not due to stomatal limitation since the girdled leaves remained turgid and their photosynthetic rate was not restored by 4% CO2. After 24 h, the girdled leaves became red through accumulation of anthocyanins and reduction of chlorophylls. A possible relationship between carbohydrate accumulation in girdled leaves and gene expression was examined through transcript levels of the ribulose-1,5-bisphosphate carboxylase oxygenase small-subunit (rbcS), the leaf-specific ADPglucose pyrophosphorylase small-subunit (Agp-l2), and Bz1, an anthocyanin pathway-specific gene. The mRNA levels in girdled leaves were also compared with those in excised leaves placed on sucrose or sorbitol solutions. rbcS mRNA disappeared 8 h after the girdling treatment and after 1 day on sucrose. Agp-l2 expression was reduced by two-thirds with girdling or sucrose feeding while Bz1 expression was strikingly enhanced by both treatments. Thus, carbohydrate accumulation produced a short-term feedback inhibition of photosynthesis and some long-term modifications in gene expression.